1. Field of the Invention
The present invention relates to surface acoustic wave (SAW) devices exhibiting improved resonance characteristics in a high frequency band.
2. Description of the Related Art
SAW devices are electronic components formed by utilizing SAWs that propagate by allowing mechanical vibration energy thereof to concentrate only on and near the surfaces of solids. SAW devices form filters, resonators, duplexers, etc.
As mobile communication terminals, for example, cellular telephones, are rapidly becoming smaller and lighter, small electronic components installed in such mobile communication terminals are also demanded.
The typical configuration of SAW devices is as follows. A pair of comb-like electrodes (hereinafter also referred to as interdigital transducer electrodes (IDT electrodes)) made of a conductive material having a small specific gravity are disposed on the surface of a piezoelectric substrate such that they face each other while the comb-like portions of the pair of IDT electrodes are interdigitated with each other. SAW devices having such a simple configuration are very suitable for reducing the size of filters, resonators, and duplexers installed in mobile communication terminals.
In known SAW devices, as a material for the piezoelectric substrate exhibiting a high excitation efficiency and a small SAW insertion loss in a high frequency band, a LiTaO3 monocrystal-line 36°-rotated X-propagating Y-cut plate is used.
When the SAW frequency is a few hundred megahertz or lower, SAW devices formed by piezoelectric substrates made of LiTaO3 monocrystal-line 36°-rotated Y-cut plates exhibit a high excitation efficiency and a small SAW propagation loss.
However, SAW devices formed by piezoelectric substrates made of the above-described type of plate are not necessarily suitable in the GHz band, which is now commonly used in, for example, cellular telephones.
Japanese Unexamined Patent Application Publication No. 2001-251157 (page 4, FIGS. 7 and 8, page 5, FIG. 12, and page 6) discloses the following SAW device. In operating in the GHz band, when the thickness of the electrodes is increased, the apparent electromechanical coupling coefficient is enhanced so as to achieve a filter with a wider band. On the other hand, bulk waves radiating from the electrodes to the inside of the substrate are intensified, thereby disadvantageously increasing the SAW propagation loss. That is, when operating in the GHz band, the advantages obtained by increasing the mass of the electrodes are noticeable.
The above publication also discloses that, to achieve the above-described advantages, the cut angle θ of the LiTaO3 monocrystal, which is used as the material for the piezoelectric substrate, is set to be larger than 36°, and more specifically, 39° to 46°, in order to minimize the propagation loss of the SAW device.
As a material for IDT electrodes of known SAW devices, Al or an alloy essentially consisting of Al, which exhibits conductive characteristics and has a small specific gravity, is generally used.
A high resistance to power is required for a SAW device used for an antenna duplexer of a radio frequency (RF) unit disposed at a stage subsequent to a transmission amplifier since high power is applied to the antenna duplexer. Additionally, as the frequency used in mobile communication terminals is increasing, it is demanded that the operation frequency of SAW devices be raised from a few hundred megahertz to a few gigahertz.
Accordingly, instead of forming IDT electrodes by Al or an alloy essentially consisting of Al, the use of Cu or an alloy essentially consisting of Cu has been suggested.
For example, Japanese Unexamined Patent Application Publication No. 2002-26685 (page 3, FIG. 1) discloses that IDT electrodes of SAW devices are formed of Cu or an alloy essentially consisting of Cu, which exhibits low electrical resistance and high resistance to stress migration.
However, only FIG. 8 and the corresponding paragraphs [0035] and [0036] of the specification of Japanese Unexamined Patent Application Publication No. 2001-251157 disclose that a SAW device was actually formed, and then, the relationships between the minimum insertion loss and the cut angle θ of the LiTaO3 monocrystal, which is used as the material for the piezoelectric substrate, were examined. The results were obtained by using the SAW filter shown in FIG. 7 of that publication when the film thickness of the IDT electrodes was set to be about 0.4 μm, which corresponds to 10% of the wavelength of SAWs to be excited.
In this publication, the relationship between the electrode thickness and the insertion loss of the SAW filter is indicated in FIG. 12 and the corresponding paragraphs [0046] and [0047] of the specification. However, the results shown in FIG. 12 are merely calculated values of the insertion loss assuming that the IDT electrodes are formed of Al or an Al(99%)-Cu(1%) alloy, and they are not based on an actual experiment.
Paragraph [0050] of this publication also discloses a preferable range of the electrode thickness when the IDT electrodes are formed of Cu. However, this publication neither discloses experimental values nor calculated values which can be a basis of this thickness range.
Accordingly, when forming a SAW device by using a LiTaO3 monocrystal as the material for a piezoelectric substrate and by using Cu or a Cu alloy as the material for the electrodes, the exact relationship between the cut angle θ of the LiTaO3 monocrystal and the electrode thickness for obtaining excellent characteristics of the SAW device has not been determined.